The present invention relates to a method of increasing the conductivity of non-single crystalline, silicon films used in making semiconductor devices, and more particularly, to increasing the conductivity of ion-implanted non-single crystalline silicon films.
In the field of semiconductor devices, particularly high-density integrated circuits, the trend has been to the use of polycrystalline silicon as a conductor, such as for making contact to regions in the silicon substrate; as interconnects between various regions of the integrated circuit, and as the gate for MOS field effect transistors. Since polycrystalline silicon itself is not conductive, it is doped with a conductivity modifier, such as boron, arsenic or phosphorus, to make it conductive. The polycrystalline silicon can be doped by either diffusing the conductivity modifier into the polycrystalline silicon film or by ion-implantating followed by annealing. Doping the polycrystalline silicon film by ion-implantation has become more popular than diffusion because it often means fewer processing steps and/or lower processing temperatures. However, it has been found that using ion-implantation for doping polycrystalline silicon films, it is difficult to obtain sheet resistivities below 40-50 ohms/square, which is higher than can be achieved by doping with diffusion. It is believed that this is because a large fraction of the implanted ions diffuse to the polycrystalline silicon grain boundaries upon post-implant annealing, there being trapped and therefore not able to contribute to electrical conducivity. More recently, in many devices and circuit applications, amorphous silicon has come into use as the starting material to form the polycrystalline silicon upon annealing the amorphous silicon and has been found to have the same problem when ion-implanted and annealed.